Permanently implantable fixture means for prothesis and the like

ABSTRACT

As a permanently implantable fixture means for prosthesis and the like in a human body, especially for implanting a dental prosthetic structure, a means composed by at least two parts has been proposed for attaching the prosthetic structure to bone tissue. A first part of the attachment unit is intended to be applied in such a way in the bone tissue that it will remain embedded in said tissue during all of the time required for healing the damage which occurs when the first part is attached to the bone and while the bone tissue grows into the attachment, and a second part of the attachment unit, serving as mounting unit for the prosthetic structure and so shaped that it may be attached to the first part of the attachment unit and pass through such weak tissue covering the bone tissue. The mounting unit is intended to combine the attachment unit inserted into the bone tissue with a prosthetic structure located outside of the weak tissue. Such permanently implantable attachment means have been used for permanently attaching dental prosthetic structure to the jawbone in human bodies. The jawbones rather often have a narrow crosssection, and the longitudinal direction of the cross-section may be diverge sharply at different parts of the same jaw and/or the same jawbone but especially between the upper jaw and the lower jaw. This, among other reasons, results in so-called over-bite or under-bite, respectively. As the attachment units must, for technical reasons, be well centered in the jawbone, the longitudinal directions of the attachment units will not always be in alignment mutually in the same jaw, nor between the upper jaw and the lower jaw. This is a serious disadvantage, because the longitudinal direction of the teeth in the prosthetic structure should be in alignment with the longitudinal direction of the attachment unit. A compromise between these demands either a weak or completely unsatisfactory attachment in the jawbone or non-aligned prosthetic structure. Therefore, there is a need for an attachment means of the kind indicated above, in which the said first part attached to the jawbone may form an angle with the said second part to which the prosthetic structure is attached.

nited States Patent 1 Bostrom [54] PERMANENTLY IMPLANTABLE FIXTURE MEANSFOR PROTHESIS AND THE LIKE Bertil l. Bostrb'm, Taby, Sweden [73]Assignee: AGA Aktiebolag,

Stockholm, Sweden [22] Filed: Mar. 12, 1971 [21] Appl. N0.: 123,707

[75] Inventor:

Ligingo near Primary ExaminerRobert Peshock Altorney- Larson, Taylor &Hinds [57] ABSTRACT As a permanently implantable fixture means forprosthesis and the like in a human body, especially for implanting adental prosthetic structure, a means composed by at least two parts hasbeen proposed for attaching the prosthetic structure to bone tissue. Afirst part of the attachment unit is intended to be applied in such away in the bone tissue that it will remain embedded in said tissueduring all of the time required 1 3,732,621 [451 May 15,1973

for healing the damage which occurs when the first part is attached tothe bone and while the bone tissue grows into the attachment, and asecond part of the attachment unit, serving as mounting unit for theprosthetic structure and so shaped that it may be attached to the firstpart of the attachment unit and pass through such weak tissue coveringthe bone tissue. The mounting unit is intended to combine the attachmentunit inserted into the bone tissue with a prosthetic structure locatedoutside of the weak tissue.

Such permanently implantable attachment means have been used forpermanently attaching dental prosthetic structure to the jawbone inhuman bodies. The jawbones rather often have a narrow cross-section, andthe longitudinal direction of the cross-section may be diverge sharplyat different parts of the same jaw and/or the same jawbone butespecially between the upper jaw and the lower jaw. This, among otherreasons, results in so-called over-bite or under-bite, respectively. Asthe attachment units must, for technical reasons, be well centered inthe jawbone, the longitudinal directions of the attachment units willnot always be in alignment mutually in the same jaw, nor between theupper jaw and the lower jaw. This is a serious disadvantage, because thelongitudinal direction of the teeth in the prosthetic structure shouldbe in alignment with the longitudinal direction of the attachment unit.A compromise between these demands either a weak or completelyunsatisfactory attachment in the jawbone or non-aligned prostheticstructure. Therefore, there is a need for an attachment means of thekind indicated above, in which the said first part attached to thejawbone may form an angle with the said second part to which theprosthetic structure is attached.

20 Claims, 7 Drawing Figures PATENTEU MAY 1 51975 Fig.6

SHEET '4 UF 4 Fig.7

PERMANENTLY IMPLANTABLE FIXTURE MEANS FOR PROTHESIS AND THE LIKE Thepresent invention refers to an arrangement in attachment means of theabove indicated kind, by which said need is satisfied.

According to the invention, there is provided between the part of theattachment means intended to grow together with the bone tissue, on theone hand, and the part of the attachment means which carries up theprosthetic structure on the other hand, a ball joint like arrangement,comprising at least one joint but perhaps two or more joints.

Further details of the invention will be apparent from the-followingdescription in connection with the attached drawings, showing a numberof different forms of execution of the invention. However, it isunderstood that the invention shall not be limited to one or more or allof these forms of execution, but that all different modifications orcombinations between the different forms of execution may be made withinthe different parts of the same jaw and/or the same jaw bone but infirst place between the upper jaw and the lower jaw. This, amongstothers, results in what is called overbite or under-bite, respectively.As the attachment units have for load technical reasons to be wellcentered in the jaw bones, the longitudinal directions of the attachmentunits will not always be in agreement mutually in the same jaw, norbetween the upper jaw and the lower jaw. This means a seriousdisadvantage, because the longitudinal direction of the teeth in theprothesis should be in agreement with the longitudinal direction of theattachment unit. A compromise between these demands causes a weak orcompletely dissatisfactory attachment in the jaw bone or that the toothprothesis will be wrongly directed.

Therefore, there is a need for an attachment means of the kind indicatedabove, in which the part, growing together with the jaw bone, may forman angle with the part, to which the prothesis should be attached.

The present invention refers to an arrangement in attachment means ofthe above indicated kind, by which said need is satisfied.

According to the invention, there is provided between the part of theattachment means intended to grow together with the skeleton tissue, onthe one hand, and the part of the attachment means, which shall carry upthe prothesis, on the other side, a ball joint like arrangement,comprising at least one joint but perhaps two or more joints.

Further details of the invention will be apparent from the followingdescription in connection with the at tachmed drawings, showing a numberof different forms of execution of the invention. However, it isunderstood that the invention shall not be limited to one or more or allof these forms of execution, but that all different kinds ofmodifications or combinations between the different forms of executionmay be made within the frame of the invention.

In the drawings,

FIG. 1 shows an arrangement according to the invention, in which onlyone single gall joint appears, and in which the ball is applied on thepart of the attachment means intended to grow' into the bone tissue,whereas the ball claw cooperating with the ball is applied on the partto which the prosthetic structure is intended to be attached.

FIG. 2 shows an arrangement in which the two means contained in the balljoint are applied in the opposite way, and

FIG. 3 shows the same arrangement as in FIG. 2, but doubled so thatthere are two ball joints in cascade. In all of the said three forms ofexecution there is a ball claw for clamping the ball.

FIG. 4, however, shows an arrangement in which the ball claw has beenreplaced by a stretch joint arrangement, running through part of thecontact surface of the ball joint, and

FIG. 5 shows the corresponding arrangement with a spherical ball.

FIGS. 6 and 7 show a variation of the arrangement according to FIG. 2.

In the arrangement according to FIG. 1 the part growing together withthe jawbone comprises a bolt formed part 10, which is provided withoutside threads for screwing the same into a hole drilled in the jawboneand provided with inside threads. The part 10 further is provided with awell 11 entering from below, which communicates through radiallydirected openings 12 with the outside of the part 10 which is providedwith the threads 13. The holes are intended for making it possible forbone tissue to grow in, in connection with the part 10 growing togetherwith the bone tissue. At the upper end the part 10 is provided with aflange 14, having an L-formed section for the reception of a free collar15, which forms, on its inner side, the socket for reception of the ball16 in the above mentioned ball joint. Hereinafter, the term socket shallbe understood as any means for retaining a ball in its positioncontained in the ball joint, independently of whether this part iscup-formed or perhaps for instance clawformed. However, it may happenthat a separate ball claw is applied between the ball and the socket.This ball claw initially is open, but is staved after the introductionof the ball, so that it will permanently embrace the ball. The ball clawmay be cut up or closed all around.

The ball 16 is attached to a shaft 17, which is provided at its lowerend with threads and is screwed into the bolt-formed part 10, said partfor this purpose being provided with a well 18 having inside threads.Between the concave part of the collar 15 forming the socket, and theconvex spherical part of the ball 16, a ball claw 19 is placed, which isattached to the upper part 20 of the attachment means. This part 20 isbuilt up in such a way that it will give a good fixture for theprosthetic structure 21, but this detail does not form the subject ofthe present invention per se, and it will therefore also not be furtherdescribed here. The upper part 20, however, is drilled through with ahole 22, running in the axial direction and inwardly threaded, in thethreads of which it is possible to screw in a bolt 23 along with anarrangement for moving bolt 23 axially, for instance a screw drivergroove 24. In FIG. 1 the screw driver groove and the parts of the cavitywithin the part 20 adjacent the groove 24 have been shown to be filledwith cement, amalgame or any similar filler mass. The bolt 23, at itslower end, is provided with a preferably conical cavity 25 having such atop angle that, when the bolt is drawn in, the edges of the cavity willmesh with the ball 16 and lock this against mutual movement between theball 16 and the upper part 20 of the attachment means.

When an attachment means of the kind described above is to be attachedto the jawbone of a patient, requiring a permanently implantedprosthetic structure 21, the dental surgeon acts so that he will firstdrill, and if required provide threads in the opening in the jawbone forthe lower part of the attachment means. Thereby he can determine theaxial direction of the opening so that this opening in the jawbone willin the best possible way be centered in the jawbone, and so that it willnot get in the immediate vicinity of the surface parts of the jaw bone.In many a case, however, because of the form of the jaw bone the drilledhole runs in a direction, the axis of which forms an angle with thedirection which the upper part 20 of the attachment means must assumebasis of the form for proper alignment of the prosthetic structure. Inthe drilled and threaded hole thereafter the dental surgeon screws inthe lower part 10 of the attachment means, after which he puts atemporary sealing arrangement of some suitable kind in the mouth of thepatient and perhaps also on the threaded hole 18 in the lower part 10,and he folds over the weak tissue, folded away at the earlier treatment,in order to heal this, and also to heal the damages caused in thejawbone. Simultaneously the lower part 10 is allowed to grow togetherwith the bone tissue or other hard tissue growing together with theouter surface of the lower part 10, which has for this purpose beenrifled or in any other suitable way made uneven. Further, bone tissuewill grow into the hole 12. Screwing up the lower part 10 thereafter iseffectively prevented.

The lower part 10 thereafter is left in the way mentioned above to growtogether with the jawbone, which usually will take approximately sixweeks. After healing is completed, the dental surgeon again cuts up theweak tissue over the lower part 10, which has now been healed, and heuncovers the lower part, screwed into the threaded hole 18, along withits cover screw by folding away and/or operating away skin and gums andhe removes the cover screw and screws in the bolt 17 along with theball, after having applied the collar in its place. At this time thepart and the bolt 17 assume a straight line and are locked in thisposition by means of the screw 22. When the bolt 17 is completelyscrewed into the hole 18 so that the part 19 is in contact with thesocket 15 the screw 20 is slightly released and the part 20 is turned tothe desired direction. After the part 20 has thus been placed in thecorrect direction, is again locked in this position by means of thescrew 22, so that it cannot be moved any further. When all attachmentsare mounted according to the above, the dentist makes a print, andguided by this the tooth technician thereafter makes a prostheticstructure, which may be cemented to the part 20.

In this connection it should be observed that the attachment meansaccording to the present invention may, of course, be used forattachment of a prosthetic structure in the lower jaw as well as forattachment of a prosthetic structure in the upper jaw. Hereinafter theterms lower" and an upper part refer to the use of the attachment meansfor attachment of a prosthetic structure to the lower jaw. Normally, fora complete prosthesis, a plurality of such attachment means arerequired, but it will be apparent to the dental surgeon in any specificcase how the prosthetic structure should be distributed along thejawbone in order to give a satisfactory attachment. For that reason onlyone single attachment means is shown in the drawings. Of course, thesame attachment means may be used for attachment of a prostheticstructure, representing a single tooth.

FIG. 2 shows a variation of the arrangement according to FIG. 1. Thelower part 27 of the attachment means has been shown here onlyschematically. One may directly screw this lower part into the drilledand threaded hole in the jawbone, but it is also possible to let thelower part 27 replace the bolt 18 in the arrangement according toFIG. 1. The essential difference between the arrangements according toFIG. 1 and FIG. 2 concerns the arrangement of the ball joint.

In the arrangement according to FIG. 2 the ball claw 28 is provided in adirection upwardly and made in one piece with the lower part 27. Itencloses in itself a ball 29, which is oval in cross-section, said ballbeing provided with a threaded hole 30 for reception of a downwardlydirected pin 31, made in the same piece as the attachment pin 32 for theprosthetic structure 33. By the pin 31 being threaded on its outside forthreadedly engaging the inwardly threaded hole through the ball 29, allof the ball joint may be locked in its adjusted position by turning allof the pin construction 31 32, whereby the ball claw 28 is clampedbetween the ball 29 and the socket 15.

The attachment of the prosthetic structure 33 may in the arrangementaccording to FIG. 2 take place in the same way as already described inconnection with FIG. 1, but it may also take place in such a way that ascrew 34 is screwed down through the prosthetic structure 33 and into athreaded hole 35 in the pin 32.

FIG. 3 shows a modification of the arrangement according to FIG. 2, inwhich one has provided for the possibility that a parallel displacementmust take place between the lower and the upper part of the attachmentmeans, perhaps in connection with a change of direction. This makes itnecessary have two ball joints, which have been shown in FIG. 3 to eachof which would be made in the same way as the single ball jointaccording to FIG. 2 the two ball joints being interconnected by acentral part 32. For this reason also the same reference numerals havebeen used, however the numerals for the lower ball joint are providedwith a prime and the numerals of upper ball joint are provided with adouble prime.

The locking of the ball joint need not necessarily take place by meansof a locking bolt of the type shown in the earlier forms of execution,but it may also advantageously take place by means of a thread, a wireor the like. In this way the ball claw 19 or 28, resp., according to theFIGS. 1 3 may be avoided. An arrangement of the last mentioned type isshown in FIG. 4.

In this case, therefore, no such ball claw as shown in FIG. 1 at 19 andin FIGS. 2 and 3 at 28 required, because in place thereof a threadformed means, for instance a wire 36 has been provided. The ball isattached in FIG. 4 to the upper end of the lower part 27 of theattachment means and forms there a semi-spherical calotte with a ratherwide recess 37, which should preferably be close to or completelyfrustro-conical. From the bottom of the recess 37 a well 38 is drilledin a direction downwardly into the lower part 27 of the attachmentmeans. This well suitably is provided with threads for attachment of aplug 39 with a central bore, through which the wire 36 runs under suchcircumstances that it will be retained in a given position againstdrawing, when the plug is screwed into the well. The socket 40cooperating with the ball 16 is in this case formed at the lower end ofthe upper part of the attachment means 20, and also this concave calottesurface is provided with a recess for the wire 36, so that it may run instretched state through both of these recesses, even if there shouldexist an essential angular difference between the lower part 27 and theupper part of the attachment means.

In the upper part 20 of the attachment means there is provided somesuitable arrangement for stretching the wire 36, after the upperattachment means part 20 has been placed in correct angular position inrelation to the lower attachment means part 27. This stretching devicemay, as shown in FIG. 4, comprise a piston 41 threaded into a bore inthe part 20 running quite through it and provided with internal threads,and further being provided with a hole for the attachment of the wire36, which is widened at its upper part in the form of an inwardlythreaded well 42. The wire is attached to the hole, so that it isstretched, when the piston 41 is screwed upwardly. The attachment of theprosthetic structure 48 may suitably take place by means of a sleeve 45.

Another form of the arrangement according to FIG. 4 is shown in FIG. 5.In this case the ball 49 is not mechanically combined with the lowerpart of the attachment means, nor with the upper part thereof. It isspherical, but it contains two conical recesses opposing each other andmeeting in the center part of the ball. The conical recesses 50 and 51have, at the place of meeting, such a width that the wire 36 may passthrough the ball. The wire 36, as earlier described, is stretched atboth ends. Thus, a conventional wire lock 52 is provided in the interiorof the lower part 27 of the attachment means. This wire lock comprisestwo cupformed parts 53, 54, directed in opposite direction directionswith respect to the open, interior parts thereof, said parts beinginwardly provided with hooks for attachment with the surface of thewire, and outwardly they are conical in order to be pressed together bypressure from the inwardly conical wall of the hole through the lowerpart 27 of the attachment means. A screw spring 55 spans the spacebetween a washer 56 and the lower surface of the two cups 53, 54 inorder to press them upwardly into the narrower part of the conical holein the lower part 27. At its upper end the wire is attached to thepiston 41 by means of a hole in this, which is also conical, and inwhich the end of the wire 36 is mounted after having been widened bymeans of a wedge 57. The part 41, as was the case in the arrangementaccording to FIG. 4, is screwed into an inwardly threaded hole in theupper part 40 of the attachment means, which carries the prostheticstructure 48.

In the arrangement according to FIG. 6 the ball is substantiallysemi-spherically formed. The ball 64 is provided with a downwardlydirected pin 65, forming the seat of a screw spring 66, the other end ofwhich rests on the bottom, which is in this case completely spherical,of the recess within the ball claw 28. In other respects the arrangementis in essential parts made in the same way as the arrangement accordingto FIG. 2, and parts corresponding to each other have therefore alsobeen provided with same reference numerals. By means of the screw spring66 the ball 64 is pressed upwardly onto the ball claw 28, so that byfriction between these two parts the ball 64 will be prevented fromrotating to turn the screw connected to the part 59.

It may also happen that the dental surgeon during his work with theattachment means will have to turn certain parts while other partsshould not be turned. This especially concerns screwing in the threadedparts. For this purpose there is provided an outer six-edged profile 67on the part 58.

A further six-edge profile 68 is provided on the lower part 27 as seenfrom the arrangement according to FIG. 7, which is not shown in section.In other parts this is similarily shaped as the arrangement according toFIG. 6. The last mentioned six-edge profile is intended to be used whenscrewing the lower part 27 into the jawbone.

I claim:

1. A prosthetic device comprising:

a first part having means to be permanently implanted into a bone suchthat the bone tissue grows into the first part,

a second part including means for attaching the prosthetic structurethereon,

and a joint connecting said first and second parts, said joint being auniversal joint permitting at least limited universal movement of thesecond part relative to the first part, and said joint including meansfor locking the universal joint in any selected position within therange of said limited universal movement to prevent further relativemovement of the first and second parts,

and including means for separating said first and second parts from eachother in the vicinity of said universal joint.

2. A prosthetic device according to claim 1, in which said universaljoint is a ball and socket joint and wherein the portion of said secondpart adjacent the joint is adapted to pass through the soft tissuecovering the bone tissue into which the first part is permanentlyimplanted.

3. A prosthetic device according to claim 2, wherein the ball of theball joint is attached to one of said parts and is embraced by a ballclaw which is attached to the other of said parts.

4. A prosthetic device according to claim 2, wherein the ball is atleast substantially spherical and is attached by means of a neck to itsrespective part.

5. A prosthetic device according to claim 2, said ball being formed asan elipsoid, ovaloid or the like, and including a pin connecting saidball to the first part.

6. A prosthetic device according to claim 2, said ball beingsubstantially semi-spherical and formed integrally with one of saidparts and the socket embracing the semi-spherical surface and attachedto the other of said parts, and including a stretching means runningthrough the ball and socket joint and urging said ball and sockettogether.

7. A prosthetic device according to claim 2, said ball beingsubstantially spherical and resting in sockets in each of the two saidparts.

8. A prosthetic device according to claim 3, said ball claw having anopening, and a pin connected to the ball and extending through saidopening and connected to the part other than that part to which the ballclaw is connected.

9. A prosthetic device according to claim 6, wherein said stretchingmeans includes at least one substantially conical recess passing throughthe ball for carrying the said stretching means.

10. A prosthetic device according to claim 9, wherein the ball issubstantially spherical and includes a pair of inwardly convergingconical recesses which meet at the center of the ball to form an openingcompletely through the ball for said stretching means.

11. A prosthetic device according to claim 2, wherein said locking meanscomprises a spring means for urging the ball against its socket.

12. A prosthetic device according to claim 11, including a ball clawembracing the said ball, and wherein said locking means includes athreaded engagement between a pin integral with one of said parts andsaid ball for displacing the ball relative to the ball claw for urgingthese two parts together.

13. A prosthetic device according to claim 11, wherein said lockingmeans includes stretching means running through the ball and socketjoint, the stretching means being attached at one of its ends to one ofsaid first and second parts, and the other end of the stretching meansbeing attached to the other of said first and second parts, and at leastone of said parts being displaceable by means of a screw threadarrangement in the axial direction under the stretching of stretchingmeans.

14. A prosthetic device according to claim 3, wherein the ball claw isprovided on its outer surface with a profile which facilitates engagingthe ball claw to turn the same.

15. A prosthetic device according to claim 3, including a sleevecontacting the outer side of the ball claw with a partially concavesurface to form said socket, and a spring means for urging the ball andsleeve together.

16. A prosthetic device according to claim 15, said sleeve beingprovided on its outside surface with a profile to facilitate turning thesame.

17. A prosthetic device according to claim 3, the said ball in the balljoint being incompletely spherical, a spherical portion of the ballengaging the ball claw, and a non-spherical portion of the ball engaginga spring arranged to urge the ball against the ball claw.

18. A prosthetic device according to claim 17, said ball including a pinfor connecting the ball with one of said parts, said pin forming a seatfor said spring.

19. A prosthetic device according to claim 18, said claw being generallyspherical and embracing the ball, said spring resting against aspherical portion of said ball claw.

20. A prosthetic device according to claim 2, including an intermediatepart, a first ball and socket joint connecting said intermediate part tosaid first part and a second ball and socket joint connecting saidintermediate part to the said second part.

1. A prosthetic device comprising: a first part having means to be permanently implanted into a bone such that the bone tissue grows into the first part, a second part including means for attaching the prosthetic structure thereon, and a joint connecting said first and second parts, said joint being a universal joint permitting at least limited universal movement of the second part relative to the first part, and said joint including means for locking the universal joint in any selected position within the range of said limited universal movement to prevent further relative movement of the first and second parts, and including means for separating said first and second parts from each other in the vicinity of said universal joint.
 2. A prosthetic device according to claim 1, in which said universal joint is a ball and socket joint and wherein the portion of said second part adjacent the joint is adapted to pass through the soft tissue covering the bone tissue into which the first part is permanently implanted.
 3. A prosthetic device according to claim 2, wherein the ball of the ball joint is attached to one of said parts and is embraced by a ball claw which is attached to the other of said parts.
 4. A prosthetic device according to claim 2, wherein the ball is at least substantially spherical and is attached by means of a neck to its respective part.
 5. A prosthetic device according to claim 2, said ball being formed as an elipsoid, ovaloid or the like, and including a pin connecting said ball to the first part.
 6. A prosthetic device according to claim 2, said ball being substantially semi-spherical and formed integrally with one of said parts and the socket embracing the semi-spherical surface and attached to the other of said parts, and including a stretching means running through the ball and socket joint and urging said ball and socket together.
 7. A prosthetic device according to claim 2, said ball being substantially spherical and resting in sockets in each of the two said parts.
 8. A prosthetic device according to claim 3, said ball claw having an opening, and a pin connected to the ball and extending through said opening and connected to the part other than that part to which the ball claw is connected.
 9. A prosthetic device according to claim 6, wherein said stretching means includes at least one substantially conical recess passing through the ball for carrying the said stretching means.
 10. A prosthetic device according to claim 9, wherein the ball is substantially spherical and includes a pair of inwardly converging conical recesses which meet at the center of the ball to form an opening completely through the ball for said stretching means.
 11. A prosthetic device according to claim 2, wherein said locking means comprises a spring means for urging the ball against its socket.
 12. A prosthetic device according to claim 11, including a ball claw embracing the said ball, and wherein said locking means includes a threaded engagement between a pin integral with one of said parts and said ball for displacing the ball relative to the ball claw for urging these two parts together.
 13. A prosthetic device according to claim 11, wherein said locking means includes stretching means running through the ball and socket joint, the stretching means being attached at one of its ends to one of said first and secoNd parts, and the other end of the stretching means being attached to the other of said first and second parts, and at least one of said parts being displaceable by means of a screw thread arrangement in the axial direction under the stretching of stretching means.
 14. A prosthetic device according to claim 3, wherein the ball claw is provided on its outer surface with a profile which facilitates engaging the ball claw to turn the same.
 15. A prosthetic device according to claim 3, including a sleeve contacting the outer side of the ball claw with a partially concave surface to form said socket, and a spring means for urging the ball and sleeve together.
 16. A prosthetic device according to claim 15, said sleeve being provided on its outside surface with a profile to facilitate turning the same.
 17. A prosthetic device according to claim 3, the said ball in the ball joint being incompletely spherical, a spherical portion of the ball engaging the ball claw, and a non-spherical portion of the ball engaging a spring arranged to urge the ball against the ball claw.
 18. A prosthetic device according to claim 17, said ball including a pin for connecting the ball with one of said parts, said pin forming a seat for said spring.
 19. A prosthetic device according to claim 18, said claw being generally spherical and embracing the ball, said spring resting against a spherical portion of said ball claw.
 20. A prosthetic device according to claim 2, including an intermediate part, a first ball and socket joint connecting said intermediate part to said first part and a second ball and socket joint connecting said intermediate part to the said second part. 